The present invention relates generally to implantable battery-operated medical devices employed as neurostimulators for treating or controlling medical, psychiatric or neurological disorders by application of modulating electrical signals to a selected nerve or nerves of the patient, and more particularly, to improved methods and devices for activating a neurostimulator to treat an epileptic attack in response to the sensing of a natural activity of the body indicative of onset of such an attack.
Extra-physiologic electrical stimulation of the vagus nerve for treatment of epilepsy and various forms of involuntary movement disorders is disclosed in U.S. Pat. No. 4,702,254 to J. Zabara (referred to herein as "the '254 patent"), exclusively licensed to the assignee of the present application. A device constituting an implantable NeuroCybemetic Prosthesis (NCP.TM., a trademark of Cyberonics, Inc. of Houston, Tex.) generator activates or controls certain nerves of the patient--for example, by use of neurocybernetic spectral discrimination in which the external current of the NCP generator is tuned to the electrochemical properties of a specific group of inhibitory nerves that affect the reticular system of the brain. These nerves may be embedded within a bundle of other nerves, and are selectively activated directly or indirectly by such tuning of the NCP generator to augment states of brain neural discharge, by which to control convulsions or seizures. According to the '254 patent, the spectral discrimination analysis dictates that certain electrical parameters of the NCP generator are to be selected based on the electrochemical properties of the nerves desired to be activated.
An improved implantable neurostimulator device is disclosed in U.S. Pat. No. 5,154,172 to R. S. Terry, Jr. et al. (referred to herein as "the '172 patent"), assigned to the assignee of the present application. The disclosure of the '172 patent is incorporated in its entirety by reference herein. In FIG. 1 of the accompanying drawings, a stimulus generator 25 and associated lead/electrode system 16/15 of a neurostimulator device 10 of the type generally disclosed in the '172 patent (but with certain improvements in sensing electrodes described in the detailed description of the invention below) are shown implanted in a patient.
Referring to the specifics of FIG. 1, the stimulus generator 25 portion of device 10, which is generally of thin circular, oval, or rectangular shape and suitably sized for implantation, is typically implanted within a surgically-formed pocket just below the skin in the left pectoral region of a patient 12. The back side of stimulus generator 25 (or the front side, depending on the implanting physician's preference as to the direction in which an electrically conductive insulatively sheathed lead 16 of the neurostimulator device 10 will extend for implantation of electrode array 15 of the lead on the vagus nerve or other selected cranial nerve of the patient) resides against the pectoral muscle in this example. The generator housing 14 (typically referred to in the art as a "can" or "case") is composed of biocompatible material (i.e., biologically compatible with the fluids and tissue of the patient's body), typically a metal such as titanium or medical grade stainless steel, and is hermetically sealed to prevent fluid penetration into the electronic components and battery(ies) (sometimes referred to herein as the "electronics package") contained therein.
A male connector at the proximal end of lead or lead assembly 16 is inserted into a female connector in a header 51 on case 14, to electrically connect the nerve stimulating electrode array 15 at the distal end of lead 16 to the proper node(s) of the electrical circuitry of the electronics package in the stimulus generator. The electrode array is preferably a bipolar stimulating electrode assembly, for example, of the type described in U.S. Pat. No. 4,573,481 to Bullara. The electrical output pulse waveform of stimulus generator 25 is applied through the lead-electrode system to a selected cranial nerve or nerve bundle of the patient on which the electrode array has been implanted, such as in the cervical location of the vagus nerve shown in FIG. 1.
The implanted neurostimulator device communicates by telemetry with a programmer and/or monitor (sometimes referred to herein as the "program console") external to the patient's body, by asynchronous serial communication, to selectively control and detect operating states of the device. Conventional external components employed for such purposes may include, for example, a programming wand 18 which transmits parameter changes to device 10 and receives device parameter and signal information to be monitored, in conjunction with computer 20 of the program console. Conventional software installed in the computer facilitates physician-controlled adjustment of selected parameters and of communication with the implanted device.
A simplified block diagram of the stimulus generator 25 of implantable device 10 is illustrated in FIG. 2. The stimulus generator includes battery(ies) 32, such as a lithium carbon monofluoride cell, electrically connected to the input of a voltage regulator 33, which powers the device. The regulated output voltage is supplied to a logic and control section 35 and other electronic sections, including a microprocessor 36 that implements and controls the programmable functions of the device. Programmable functions may include the magnitude of current or voltage, the frequency, the pulse width, and the on-time and off-time of output pulses generated by the stimulus generator for application to the lead assembly and thence to the distal electrode array and the nerve on which it is implanted. The programmability of the device enables the attending physician to selectively tailor its output pulse waveform to modulate the electrical activity of the vagus nerve to provide a prescribed therapy regimen for the disorder being treated--epilepsy, in this case. Timing of the logic and control and other functions of the stimulus generator is controlled by a precise output frequency signal of a crystal oscillator 37. A magnetically-actuatable reed switch 39 enables the patient to manually activate the generator for initiating the delivery of its output pulses to the nerve by means of an external magnet (not shown).
Built-in antenna 40 is provided for use in bidirectional telemetry communication between the implanted stimulus generator and the external electronics of the program console, for supplying the programming signals necessary to set or change the output pulse parameters, and to detect device operation, via wand 18 (FIG. 1). Once the generator is programmed, it operates continuously at the programmed settings until they are re-programmed (by the attending physician) by means of the external program console.
Logic/control section 35 controls output circuit 42 for producing the output pulse waveform according to the prescribed therapy. When the stimulus generator is activated, which may be continuously or periodically to provide prophylactic/preventive treatment of the disorder, or which may be controlled by the patient's manual activation or by automatic activation upon sensing physiologic changes indicative of onset of the disorder, the programmed output waveform is delivered via the electrical connector in the header of the generator case to lead assembly 16 and stimulating electrode array 15 (FIG. 1). This electrical stimulation produces a predetermined modulation of the electrical activity of the nerve on which the electrodes are implanted to treat, control, and alleviate the disorder. Where the patient is being treated for epileptic seizures, for example, the electrical activity of a cranial nerve such as the vagus nerve is modulated by initiation of the stimulus waveform in response to a set of detection criteria having been met, for the purpose of inhibiting or interrupting a seizure, or of lessening the frequency or severity of seizures.
Certain techniques of manual and automatic activation of implantable medical devices are disclosed in U.S. Pat. No. 5,304,206 to R. G. Baker, Jr. et al. (referred to herein as "the '206 patent"), which is assigned to the same assignee as the present application. The '206 patent discusses problems that may be encountered when a patient seeks to manually activate the device upon sensing onset of the disorder, such as quickly locating the magnet; as well as the surgical procedural problems experienced where special sensors such as EEG electrodes are sought to be implanted for automatic activation. According to the invention disclosed in the '206 patent, means for manually activating or deactivating the stimulus generator include a sensor such as an accelerometer or a piezoelectric element mounted to the inner surface of the generator case and adapted to detect light taps by the patient on the implant site. In this way, the patient is given limited but convenient control over the device operation, to an extent which will be determined by the attending physician. The '206 patent also discloses an automatic activation technique for the implanted device, in which random jerky movements of the patient that occur with some types of seizures, and which are not consistent with normal activity, are detected by a vibration sensor, accelerometer, or other means having programmable sensitivity. The sensor may thus be fine-tuned to the seizure characteristics of a particular patient without likelihood of being triggered by normal activity of the patient.
The present invention is directed toward further improvements in automatic activation of an implanted neurostimulator device, particularly for treating epilepsy. A principal objective of the invention is to provide apparatus and methods for activation of an implanted stimulus generator for treating epileptic seizures by sensing changes in certain physiological parameters which can be a reliable precursor of an oncoming attack.